The technology of intra-core fibre Bragg gratings has spawned many new devices that have found applications in the telecommunication and optical sensing industries. These gratings are used to perform filter functions in the optical domain that are simply not possible (or cost-effective) in the electronic domain. Fibre Bragg gratings are playing an important role in the optical communication technology of wave division multiplexing. In one particular example, the application of chirped fibre Bragg gratings to chromatic dispersion compensation has become an important consideration in optical links as the data rates increase to meet market demands. Further, chromatic dispersion compensators based on fibre gratings are now commercially available. Accordingly, the fabrication and characterisation of fibre Bragg gratings has become of significant importance in the optical telecommunication market.
Fibre Bragg gratings have very similar characteristics to classical free space diffraction gratings where, for a nominal angle of incidence, the reflectivity is primarily a function of the incident wavelength. Fibre Bragg gratings can thus be considered a subset of the free-space diffraction gratings. The distinction, however, is that the angle of incidence in fibre gratings is fixed to being perpendicular to the grating length (assuming the grating cross-section is perpendicular to the fibre waveguide). The Bragg condition is satisfied when the partial reflectors that make up the grating contribute a reflected component of the incident electric field in phase with all the other partial reflectors in the grating. Accordingly, the Bragg wavelength is expressed as:λB=2{overscore (n)}Λ,  (1)where                λB is the Bragg wavelength (m),        {overscore (n)} is the average refractive index, and        Λ is the pitch of the grating (m).        
As the wavelength is de-tuned away from the Bragg condition, the phase synchronicity between the ensemble of reflections rapidly deteriorates and the constructive interference ceases. In other words, the resonance condition rapidly ceases with increasing distance either side of the Bragg wavelength. A typical grating spectral response yields an extremely narrow band of wavelengths where the resonant reflection exists, so they represent a nearly ideal band-pass or notch filter in the optical domain.
An existing Michelson interferometer relevant to the present invention is shown schematically in FIG. 1. The interferometer has two arms of different lengths, of path imbalance ΔL. In this Michelson interferometer, laser light is input along fibre 10 and transmitted to a beam splitter in the form of coupler 12, where the light is split along arms 14 and 16 of length, respectively, L and L+ΔL. Light is reflected by respective mirrors 18 and 20, and transmitted back to coupler 12. The light—in fact as interference fringes—is then observed by means of a photodiode 24 located at the exit of output fibre 22.
In uniform fibre Bragg gratings the spacing between successive partial reflectors is fixed throughout the grating. However, it is possible to fabricate fibre Bragg gratings in which the spacing is not fixed. This is the case in, for example, with linearly chirped fibre Bragg gratings, in which the spacing between successive partial reflectors changes at a constant rate over the length of the grating. Chirped and other non-uniformly spaced gratings have a number of interesting properties and advantages. For example, the linear chromatic dispersion of single mode optical fibre has become one of the limiting factors in exploiting the intrinsically large bandwidth that optical fibre communications offer. Chromatic dispersion is a result of both material and waveguide dispersion, which are in turn a function of the wavelength dependence of the effective refractive index and the profile of the core's refractive index respectively. The effect of chromatic dispersion may be illustrated with a transmitted optical pulse containing a finite band of wavelengths. As this pulse propagates through the optical fibre, the chromatic dispersion effectively advances the longer wavelength constituents and retards the shorter wavelengths. This induces the edges of the pulse to dilate in time which, in the context of a communication bit stream, may interfere with the adjacent bit cell. A typical figure for the dispersion in communication grade fibre is approximately −16 ps.nm−1.km−1.
Chirped fibre Bragg gratings can be designed to exhibit high degrees of linear chromatic dispersion. With a linear chirp in the pitch across a grating, the apparent reflection point varies linearly with wavelength. Thus, for a pulse that has been dilated through chromatic dispersion in optical fibre, it is possible to recompress the pulse by reflecting it off a correctly orientated Chirped fibre Bragg grating. It is possible to achieve 1000 ps.nm−1 of chromatic dispersion in a grating of a length 10 cm. Thus, it is possible to compensate optical fibre dispersion that occurred over many hundreds of kilometres using a very short low loss device.
As a consequence of the non-uniform spacing, the Bragg condition varies as a function of the spatial position within the grating. Also, whereas in a uniform grating the effective reflection point from within the grating is approximately constant with small detunings, small wavelength detunings in chirped gratings induce a movement in the Bragg resonance, the result of which is a change in the apparent reflection point from within the grating. Consequently, such gratings have found a number of important applications, so it has become important to be able to characterize accurately the properties, such as chromatic dispersion, of such gratings.